Expanded packing products, which are hardened foams, enjoy widespread use as loose-fill packing material. Loose-fill packing based on polystyrene constitutes a major market for expanded foam packing products. Such plastic foams have several drawbacks however. Firstly, their methods of manufacture are complex and therefore expensive. Secondly, the expansion (foaming) step in their manufacture involves a blowing agent, such as pentane, which presents a fire hazard or a chlorofluorcarbon, which is environmentally unacceptable unless a still more costly hydrofluorocarbon alternative blowing agent is used instead. Furthermore, although such packing can be formulated (at added cost) to be degradable in direct sunlight, it still suffers from the serious disadvantage that it is not biodegradable or hydrolytically degradable under the environmental conditions that prevail in compost heaps, landfills, or other common disposal means.
U.S. Pat. No. 3,137,592 to Protzman discloses heating a mixture of starch and water to 125-250 C under pressure sufficient to maintain the water in the liquid state and then relieving the temperature and pressure in an extruder.
U.S. Pat. No. 4,588,638 and U.S. Pat. No. 4,644,733 to Dolinar are directed to expanded loose-fill packing comprising polymer particles (e.g. polyethylene) in which "at least a portion" of the surface of "a majority of the particles" are coated with an adhesive (such as glue, polymer latex or "starch-based adhesive") to reduce migration of the particles in use. The patents also disclose a method of packing articles in containers with such polymeric loose-fill particles.
U.S. Pat. No. 4,673,438 to Wittwer discloses injection molding starch-water mixtures which have been heated to a temperature above the T.sub.g and melting point of the composition. The water content of the mixture is maintained throughout the molding process.
Lacourse et al. U.S. Pat. No. 4,863,655 discloses a biodegradable packing material comprising an expanded high amylose starch prepared by extruding the starch in the presence of up to 21% moisture. High amylose starch is, of course, quite expensive.
U.S. Pat. No. 5,300,333 to Wilkerson et al is directed to moldable mixtures comprising expanded grains such as popcorn, which are dispersed in a continuous phase of biodegradable bonding agent, such as starch paste.
U.S. Pat. No. 5,413,855 to Kolaska et al is directed to molded bodies comprised of expanded beads of starch, poly(vinyl alcohol) and water which are placed in a closed mold and then heated with low humidity air or dry steam to dissolve the surface of the beads and make the beads stick together.
U.S. Pat. No. 5,095,054 at Lay et al. is directed to the manufacture of shaped articles by extruding a material comprising a destructurized starch and any of several classes of organic polymers and copolymers such as modified polysaccharides, poly(vinyl pyrrolidone), cationic and anionic starches, poly(vinyl acetate) polymers and copolymers and the like. The polymers are added for the purpose of increasing stability, stiffness, elasticity, etc., of the shaped articles.
While the use of loose-fill packing of the kind described above has been widely accepted, such environmentally friendly packaging has not been available for the manufacture of molded packaging shapes, such as those which are used to package electronic components and other fragile items. Heretofore, such molded packing shapes have largely been made from non-degradable synthetic polymeric materials such as polystyrene.